The present invention relates to a speed change control apparatus and method for an automotive automatic transmission, and more particularly, to a speed change control apparatus capable of reducing the delay in shift response and reducing shift shock, even when the temperature of operating oil of the automatic transmission is low.
Motor vehicles are equipped with an automatic transmission for automatically setting an optimal transmission gear ratio in order to properly transmit power generated by an engine to drive wheels in accordance with vehicle operating conditions. A typical automatic transmission for vehicles comprises an input shaft and an output shaft coupled, respectively, to an engine output shaft and drive wheels, and a plurality of rotating elements (rotating drum, gear wheels, etc.) forming a power transmission path between the input and output shafts. The transmission further comprises a plurality of hydraulic frictional engaging elements (clutches, brakes, etc.) associated with the rotating elements for selectively establishing a power transmission path, i.e., a transmission gear ratio, between the input and output shafts, a speed change control apparatus responsive to various sensor outputs representing vehicle operating conditions, and an electricity-hydraulic pressure converter provided between the control apparatus and the frictional engaging elements. The speed change control apparatus includes a memory for storing a shift diagram in which a plurality of shift lines are individually set as a function of a state parameter (e.g., throttle opening and vehicle speed) relating to the vehicle operating conditions. To prevent unstable travel caused by frequent shifting near a shift point at which shift is effected, there is provided a difference (hysteresis) between the shift point applied at the time of increase of the vehicle speed and the shift point applied at the time of decrease of the vehicle speed. Specifically, downshift lines are generally set on a lower-speed side than the corresponding upshift lines.
During operation of the vehicle, in order to establish a gear suited to the vehicle operating conditions, application of the hydraulic pressure to the frictional engaging elements is controlled by the electricity-hydraulic pressure converter under the control of the speed change control apparatus, in accordance with the vehicle operating conditions. For example, during a shift from a higher-speed gear to a lower-speed gear, the frictional engaging elements for establishing the higher-speed gear are disengaged while the frictional engaging elements for establishing the lower-speed gear are engaged. As a result, only the required ones of the rotating elements are selected and rendered operative so as to form a power transmission path, whereby the gear transmission ratio between the input and output shafts of the transmission is changed to establish the necessary gear.
Typically, the frictional engaging element has friction members for engagement and disengagement with respect to each other, a piston actuated by hydraulic pressure for pressing the friction members against each other, a cylinder containing the piston, etc. When the frictional engaging element is in a disengaged state, the piston assumes a standby position where there is a sufficient clearance between the piston and the friction members, thereby preventing drag torque. When engaging the frictional engaging element, hydraulic pressure is introduced into the cylinder such that the piston at the standby position is first moved to an engagement start position at which the piston contacts the friction member. In other words, the piston is moved over an ineffective stroke from the standby position to the engagement start position to eliminate the dead space. Thereafter, hydraulic pressure is further applied to gradually engage the friction members with each other. To quickly carry out the shift operation, a maximum hydraulic pressure is introduced into the cylinder while the piston moves through the ineffective stroke, to thereby eliminate the dead space in the shortest possible time.
If, during travel of the vehicle, the accelerator pedal is frequently depressed and released, the operating point in the shift diagram frequently crosses the upshift or downshift line based upon change in the throttle opening. Thus, an upshift or downshift takes place each time the upshift or downshift line is crossed. Meanwhile, when the temperature of the operating oil of the automatic transmission is low, the viscosity of the operating oil is large. Thus, the "release" and "application" of the hydraulic pressure is prolonged, lowering the hydraulic pressure response, the output hydraulic pressure characteristic of solenoid valves used in electronic controlled automatic transmissions, etc. Consequently, when the operating oil temperature is low, supply of the hydraulic pressure cannot follow the frequent shifting as previously mentioned, and a disadvantage arises in that the shift time is prolonged, or that shift shock occurs due to deficiency of the operating oil pressure.